The present invention relates in general to systems for evaporating a liquid and in particular to a method of and a device for evaporating and recovering water from agueous solutions.
In industrial processes, it is frequently desired to reduce the proportion of water in a liquid or to remove water from the liquid. In many cases, the extraction of water is the primary aim of such processes, nonetheless, the residual liquid is mostly also of interest. Particularly in the field of recovering valuable materials, new problems arise which necessitate the extraction of water from liquids or solutions containing such valuable materials.
For example, in the galvanization technology, efforts are made to recover dissolved valuable substances, from water which was used for rinsing. For this purpose, it is necessary either to separate such valuable substances from the water or to reduce the portion of water in the solution to such extent that upon the return of the solution into the active bath, the introduced amount of water is not larger than the losses of water occurring due to evaporation of the upper surface of the galvanizing bath. When recovery of metals from rinsing baths by means of electrolytic processes, an economic recovery can be made only upon the concentration of the solution. The application of this electrolytic method has, apart from the recovery of valuable material, the additional advantage in eliminating costs for purification or decontamination of waste water.
In active baths which operate at temperatures from about 60.degree. centigrade and more, it is possible by applying suitable rinsing technology such as a multiple cascade emersion and rinsing or a multi-phase jet rinsing to return in the bath the whole amount of the rinsing water together with valuable substances contained therein inasmuch a corresponding amount of water evaporates from the surface of the bath.
If, however, the active bath is operated at low working temperatures it is impossible to return the rinsing water since the volume of the bath would continually increase.
In view of the recovery of precious substances and the saving of costs for the treatment of the resulting waste water, it is therefore, in many cases more economical to evaporate from the bath by means of a separate device a portion of water which corresponds to the required volume of rinsing water.
In many instances during the process taking place in the active bath there results excessive heat which anyhow has to be taken off. This is the case in various electrolytic processes using agueous solutions. It is possible to combine the cooling of such baths with the evaporation of a portion of water contained therein whereby rinsing water containing valuable substances can be returned again in the bath (German Publication No. 1 962 249).
If on the other hand, the imposed task is to withdrawn water from a liquid having a low temperature level without having for disposal any energy released during the process so in conventional methods the energy applied for the evaporation of water from the solution is lost. In addition, in such prior art methods high cost equipment is necessary for the evaporating process and in many cases the temperature which is necessary for evaporating water at the pressure of the outer atmosphere is detrimental to the contained valuable substances. It is true that it is possible to reduce the temperature by reducing the pressure above the treated liquid but the cost of apparatuses necessary for applying this so called vacuum evaporation process is so high that the whole process may become uneconomical.
By applying the evaporization method in the treated bath, there are required large amounts of air which according to ambient conditions may absorb additional moisture. Inasmuch the ambient air is frequently almost saturated with moisture the air must be heated up in order to produce the condition that the heated air be capable of absorbing and carrying additional moisture. Also, in the latter case, the supplied energy is lost and the operational costs are frequently so high that they prevent the application of this known method. Another serious disadvantage of the exhaust air vaporization is the occurrence of a chemical conversion such as the formation of carbonates between the components of the air such as carbon dioxide and the solution to be concentrated for example.
It has been also proposed that the air carrying the additional moisture be not discharged directly into the outer atmosphere but that it should be guided past a condenser whereby water in pure condition is recovered and can be returned into the process (German Published application No. 2 656 103). It is true that in this method the latent heat released due to the evaporization of the moisture is transferred to the condenser while the recovered heat is on such a low temperature that by conventional methods it cannot be effectively used for the process. Apart form the recovery of water, the application of a condenser in prior art methods therefore do not bring any advantages. For operating a condenser, it is in most cases, necessary to employ also a cooling machine because the existing cool water cannot attain such low temperatures which are required for condensation of moisture from the air.